The present invention relates to a small-sized portable printer for performing printing on checks or the like.
In some conventional small-sized printers, such as printers for checks, the printing section consists of a thermal transfer printer, as disclosed in Japanese Patent Unexamined Publication No. 62-234959. The application of such conventional printers for checks has been restricted to printing on checks and bank account balance statements.
Checks are used in commercial transactions, where simplicity and speed are essential. Thus, the user of the check cannot always be expected to handle it carefully, and in many cases the check is creased and wrinkled through rough handling. Accordingly the check is rarely in an ideal condition when it enters the printing section where printing is to be performed.
In the case of a conventional check printer whose printing section consists of a thermal transfer printer, such creases and wrinkles are a problem, since they will lead to inadequate contact between the printer and the check, resulting in defective printing, which is not acceptable, since it may cause trouble in the future.
Further, conventional check printers have the function of comparing the initially input amount with the amount issued and printing and displaying them. In actual commercial transactions, however, the mode of transaction varies. For example, payment can be made by cash, cards, etc. Thus, conventional check printers are incapable of adapting themselves to such varied transaction modes, which is very inconvenient for the user.
A small-sized printer is known which performs printing on rolled paper or standard sheets by means of a shuttle head. This type of small-sized printer will be described below.
FIG. 8 is a plan view of the essential part of the printing section of a small-sized printer of the above-mentioned type. Numeral 1' indicates a printing ribbon cassette, and numeral 2' indicates a printing ribbon drawn out of the printing ribbon cassette 1'. Numeral 3' indicates a shuttle head having a hammer 4', at one end of which a pin 5' is provided. Numeral 6' indicates the axle of the hammer 4'; numeral 7' indicates a magnetic pole; numeral 8' indicates a bobbin; numeral 9' indicates a return spring; and numeral 10' indicates a coil. Numeral 11' indicates a head, and numerals 12' and 13' indicate shafts extending through the head 11'. The head 11' is driven by a drive source (not shown) to reciprocate along the longitudinal length of the shafts 12' and 13'. Numeral 14' indicates a paper feeding roller, and numeral 15' indicates an auxiliary roller which is held in elastic contact with the paper feeding roller 14' by a spring 16'. Numeral 17' indicates a standard sheet. Numeral 18' indicates a cabinet; numeral 19' indicates a paper inlet; numeral 20' indicates a paper outlet; and numeral 21' indicates a platen. In this conventional small-sized printer, the mounting positions of the shafts 12' and 13' and the head gap between the pin 5' and the platen 21' have been adjusted, thereby securing a fixed level of printing quality.
The operation of the above-described small-sized printer will now be described.
First, the standard sheet 17' is inserted through the paper inlet 19', and stopped where it abuts the paper feeding roller 14'. In this condition the paper feeding roller 14' is rotated, and the standard sheet 17' reaches the position where printing should be performed.
Next, when performing printing, electric current is applied to the coil 10' to operate the magnetic pole 7', which pushes the hammer 4', thereby causing the pin 5' to operate using the axle 6' as the fulcrum. The pin 5' is pressed against the printing ribbon 2' so as to transfer the ink, with which the printing ribbon 2' is imbued, onto the standard sheet 17'. When the supply of electric current is stopped, the hammer 4' is returned to its initial position by the return spring 9'.
The small-sized printer of the above-described type has a problem in that the paper feeding roller 14' has to be rotated by the drive source when inserting the definite-form sheet 17', which makes the setting rather difficult. To facilitate the insertion, a releasing mechanism for the auxiliary roller 15' might be provided, which, however, would result in a complicated structure and high costs. Further, since the width of the paper inlet 19' must be adapted to the maximum dimension of the prescribed sheet size, the sheet cannot be perfectly positioned along the lateral dimension thereof. Also, positioning along the paper feeding dimension cannot be effected correctly. Thus, in the case of a sheet such as a check, in which the printing position has been determined beforehand, displacement of the printing position or skewing may occur. Further, in the above-described small-sized printer, the head gap adjustment requires a great deal of skill and time. In this regard, a function might be provided by which the pressure applied to the pin 5' is measured for automatic adjustment, which, however, would involve a complicated structure and high cost.
In recent years, there is a demand for a reduction in printing time in small-sized printers. In this regard, an increase in printing speed has been attained by two-direction printing. To perform two-direction printing, it is necessary for the home position of the printing head to be detected with high accuracy so that the requisite stability in registration for printing may be ensured in both directions.
A conventional small-sized printer of this type will be described with reference to FIG. 16. Referring to the drawing, the conventional small-sized printer comprises a motor 301 for outputting driving force to different parts of the printer; a gear 302 connected with the driving shaft of the motor 301; a cylindrical cam 303 connected with the gear 302 and having a meandering guide groove 330 on its surface; a printing head 304 which moves in engagement with the guide groove 330 to effect printing in a fixed area of a sheet; a home detecting means 305 for detecting the home position of the printing head 304; a shutter 306 attached to the printing head 304; an encoder 311 connected with the motor 301 and having holes arranged at fixed intervals; a pulse generating means 308 for generating pulses by means of the encoder 311; a calculating means 390 for calculating the home position of the printing head 304 on the basis of signals from the home detecting means 305 and the pulse generating means 308; and a driving means 310 for driving the motor 301 in accordance with the calculation results to move the printing head 304 to the home position.
The operation of this conventional small-sized printer, constructed as descried above, will now be explained.
First, the cylindrical cam 303 is rotated by the motor 301 through the gear 302. The printing head 304, which has a protrusion 340 engaged with the guide groove 330 of the cylindrical cam 303, moves to the right and left to perform printing. When the printing head 304 has been moved to the home position, the shutter 306 attached to the printing head 304 intercepts the home detecting means 305 consisting of a transmission-type optical sensor, thereby generating a home detection signal. The pulse generating means 308 generates pulses at fixed intervals (FIG. 18A) in synchronism with the rotation of the motor 301, to which the encoder 311 comprising a disk 370 as shown in FIG. 17 is attached, and the calculation means 390 performs a calculation on the home detection signal and the pulse signals, whereby the home position of the printing head 304 is determined. Then, printing is started from the home position while counting the pulses generated. By this arrangement, it is always possible to start printing at a position spaced apart from the home position by a fixed distance, thereby ensuring the requisite stability for printing in one direction.
However, although the above-described conventional construction makes it possible to effect correct registration in one-direction printing, it has the following problem: In two-direction printing, it involves generation of a positional deviation between the printing head 304 and the encoder 311 due to the backlash of the cylindrical cam and the gear. Thus, as shown in FIG. 18A, assuming that the first pulse of the home detection signal corresponds to the home position, such a positional deviation leads to a pulse gap, resulting in the home position being displaced by one pulse. As a result, as shown in FIG. 18B, a positional deviation is generated in the printing performed in direction 2, resulting in the printed characters being deformed. In view of this, it is necessary for the encoder 311 and the pulse generating means 308 to be of high precision, and for the gear to exhibit no backlash. Thus, ideal two-direction printing is difficult to realize.